THE BRONCHI. 925 
mediastinum, being separated from the bodies of the vertebrae by the cesophagus 
alone. Immediately above its bifurcation the deep cardiac plexus of nerves is 
placed in front and on either side of the trachea. At the level of the fourth dorsal 
vertebra the aortic arch is very intimately related to it. At first in front of the 
tube, the aortic arch passes backwards in close contact with its left side. The 
three great vessels which spring from the aortic arch are also placed in close proximity 
to the trachea. The innominate and left common carotid arteries, at first in front, 
gradually diverge as they proceed upwards and come to le on either side of the 
Pabe—the innominate to the right, and the left common carotid to the left. In 
front of these vessels are the left innominate vein and the remains of the thymus 
body. On the right side the thoracic part of the trachea is in relation to the right 
vagus nerve, and is clothed by the right mediastinal pleura; on the left side are 
the left subclavian artery and the left recurrent laryngeal nerve. 
Structure of the Wall of the Trachea.—The wall of the trachea and bronchi is 
composed of (1) a fibro-elastic membrane in which the cartilaginous rings are embedded ; 
(2) within this, and on the posterior aspect of the tube, a layer of muscular tissue, termed 
the musculus trachealis ; and (3) the lining mucous membrane. 
The fibro-elastic membrane is strong and dense, and, passing round the whole circum- 
ference of the tube, it becomes continuous superiorly with the perichondrium which 
invests the cricoid cartilage. Embedded in its substance are the series of cartilaginous 
rings. ‘These vary in number from 15 to 20, and are composed of hyaline cartilage. 
They are horseshoe-shaped, the posterior fourth of the circumference being deficient, so 
that behind, each ring ends in two rounded extremities. The external surface of a tracheal 
ring is flat and even, and does not project much beyond the level of the membrane in 
which it is embedded ; the inner surface, however, is convex in the vertical direction, and 
consequently it bulges slightly into the Iumen of the trachea. The intervals between the 
rings are somewhat narrower than the rings themselves, and neighbouring rings frequently 
show a more or less complete fusion, whilst others present other irregularities, such as a 
tendency to bifurcate. The lowest ring is specially adapted to the ceil bifurcation. 
In the middle line in front it inclines downwards, and from this median peak a cartilaginous 
strip is carried backwards in the fork between the two bronchi. 
The musculus trachealis is a continuous layer of involuntary muscular tissue placed 
in the posterior part of the wall in front of the fibro-elastic membrane. The muscular 
bundles are arranged transversely, and are attached to the extremities of the rings, and 
also to the deep surface of the rings for a short distance beyond their extremities. Tn the 
intervals between the rings the transverse muscular bundles are attached to the fibro- 
elastic membrane. It is evident that, by its contraction, this muscle will reduce in a 
marked degree the lumen of the tube. 
The mucous membrane is laid smoothly over the interior of the tube upon a layer of 
submucous areolar tissue. Lymphoid tissue enters largely into the composition of the 
tracheal mucous membrane, and its inner surface is lined by columnar ciliated epithelial 
cells. The action of the cilia exercises an important influence in producing an upward 
movement of the mucus which is present on the surface of the mucous membrane. 
Numerous longitudinal bundles of elastic tissue are present in the posterior wall of 
the trachea, more particularly in its lower part, between the mucous membrane and the 
musculus trachealis. 
In connexion with the mucous membrane there is a plentiful supply of acinose mucous 
glands. These are placed in the submucous tissue, and also on the posterior aspect of the 
tube on the exterior of the musculus trachealis as well as amidst its muscular bundles. 
They send their ducts to the surface of the mucous membrane, where they open by 
trumpet-shaped mouths. 
\ THE BRONCHI. 
The two bronchi proceed obliquely downwards and outwards from the termina- 
tion of the trachea, each towards the hilus of the corresponding lung. Like the 
trachea they are kept permanently patent by the presence of cartilaginous rings in 
their walls. These rings are deficient posteriorly, so that the bronchi exhibit a 
flattened posterior surface in every respect similar to the trachea. The two 
bronchi differ from each other, not only in the relations which they present to sur- 
rounding structures, but also in length, in width, and in the direction which they 
pursue (Fig. 625, p. 923). 
: 
Biin® 
